Various materials such as nitrogen oxide (NOx) and particle matters (PM) are generally contained in gas (exhaust gas) discharged from combustion chambers of internal combustion engines. An amount of emission of these materials to the outside of the engine is desirably reduced as much as possible. Thus, exhaust gas purification devices to purify exhaust gas by removing these materials from the exhaust gas are conventionally designed.
For example, an exhaust gas purification device having a specific catalyst is conventionally designed, which catalyst is installed on an exhaust passage and purifies exhaust gas by selectively reducing specific components included in the exhaust gas (so-called selective catalytic reduction catalyst, hereinafter referred to as “SCR catalyst”). This kind of exhaust gas purification devices include, for example, a device having a SCR catalyst to selectively remove (reduce) nitrogen oxide (NOx) included in the exhaust gas and supplying a reduction agent (e.g., urea solution) to the exhaust gas to be guided to the catalyst. This exhaust gas purification device removes nitrogen oxide from the exhaust gas (i.e., reduces nitrogen oxide to nitrogen and water) by reacting nitrogen oxide included in the exhaust gas and the reduction agent (e.g., ammonia generated through hydrolysis of urea of the urea solution) in the SCR catalyst.
One of exhaust gas purification devices having the SCR catalyst (hereinafter referred to as “conventional device”) controls the composition of the nitrogen oxides (molar ratio between nitrogen monoxide and nitrogen dioxide) included in exhaust gas to be guided to the SCR catalyst in order to remove the nitrogen oxide from the exhaust gas as effective as possible.
In particular, the conventional device has an electric heater, an oxidative catalyst to invert nitrogen monoxide to nitrogen dioxide (the inversion rate between nitrogen monoxide to nitrogen dioxide depends on the temperature of the oxidative catalyst), a diesel particulate filter (hereinafter referred to as “DPF”) to collect particle matters, a nozzle to supply urea solution as the reduction agent in exhaust gas, and an SCR catalyst, in this order on the exhaust passage. The conventional device controls the temperature of the oxidative catalyst by changing an amount of heat generation of the electric heater (i.e., temperature of exhaust gas to be guided to the oxidative catalyst) depending on operating condition of the engine. Thereby, the conventional device controls the composition of nitrogen oxides included in exhaust gas to be guided to the SCR catalyst after passing the oxidative catalyst to a specific composition at which nitrogen oxides are effectively removed (reduced) with the SCR catalyst. For example, see the patent literature 1.